This invention relates to a circuit which controls a speed changing device.
An example of a prior circuit which controls the speed of a motor is shown in applicant's U.S. Pat. No. 3,953,776, which is assigned to the assignee of the instant invention. In the prior circuit, the speed of a motor is measured during a first fixed time interval. During this interval it is determined whether the motor is running fast or slow in comparison to a preset speed. If the motor is running faster or slower than the preset speed, its speed is corrected during a second fixed time interval which immediately follows the first time interval. Under some circumstances, as when the motor speed is quite slow, the first time interval must be relatively long, which means it takes a relatively long time to correct the speed of a slow running motor. It is desirable to have a speed control circuit which has a fast response time.
It is also desirable to have a control system in which the reference speed, or the desired speed, of a device can be set manually by a digital reference or automatically by an input from a control process or a computer.
Another problem with prior speed control circuits is that when a change is made in an attempt to bring the speed of the device closer to that of a preset reference the change causes the device to overshoot the reference speed. This is because most speed change actuators have significant inertia which prevents the cut-off of their speed change effect when the desired speed is reached. This causes the device to continually hunt for the reference speed.
In the instant invention the signal which operates a speed changing device is pulsed to allow the device to coast to the reference speed. In other words, the duration of the time of operation of the speed changing device is such that the speed of the device is changed in small increments until the reference speed is reached. This significantly eliminates overshoot of the reference speed and hunting by the device.